De-oiling, to remove residual oil and solids from process water, is an important step in various methods for producing crude oil or bitumen.
For example, one such method is Steam-Assisted Gravity Drainage (SAGD), which can be used in the production of synthetic crude oil from bitumen in oil sands, such as the Athabasca oil sands in Alberta, Canada. Oil sands are typically deposits of loose sand or partially consolidated sandstone containing bitumen. Bitumen is a type of oil having high viscosity, typically high enough that the bitumen effectively behaves like a solid and does not flow at ambient conditions. However, SAGD is not limited to the production of synthetic crude oil from oil sands bitumen, and can also be used in the production of other types of highly viscous or heavy crude oil.
SAGD typically involves at least two wells drilled into an oil deposit at different depths. Steam is injected into the deposit through the upper well to heat the oil and thereby reduce its viscosity. Gravity then tends to cause the heated, less viscous oil to drain downward toward the wellbore of the lower well, along with condensed water from the cooling steam. The condensed water and oil are then pumped out from the lower well, along with some gases that are released during the process.
An initial production treatment phase then removes much of the oil, and exhausts a flow of “produced water” which still includes significant amounts of residual oil in the form of dispersed oil droplets, as well as suspended solids.
A de-oiling train then receives the flow of produced water. The role of the de-oiling train is to remove as much of the residual oil as possible from the produced water, not only to recover the value of the residual oil, but also to permit the produced water to be recycled for steam generation, in order to reduce the consumption rate of water required to carry out the SAGD process. Excess residual oil in the produced water can cause serious problems for steam generators and their associated water treatment components, including fouling of ion exchange resin in water softeners, and damage to tubes in the steam generators themselves. This in turn can lead to costly production shut-downs or reductions, often resulting in millions of dollars worth of lost production revenue, as well as significant maintenance and repair costs.
The de-oiling train typically includes (among other components) a skim tank, for primary separation of the residual oil and suspended solids from hot produced water. The de-oiling train may also include other secondary separation components.
A skim tank is effectively a very large gravity or buoyancy separation tank. Oil droplets have a lower density than water and tend to rise in water due to buoyancy whereas denser particles tend to settle, forming a layer at the bottom of the tank. The separated oil forms an oil layer at the top of the liquid which is removed using a skimmer and piped out. The separated water enters a separate outlet pipe near the bottom of the skim tank and is exhausted from the tank.
The terminal velocity of a small oil droplet, meaning the velocity at which it will rise in water due to buoyancy, is given by Stokes' Law:Vt=gd2Δρ/18μ                where:                    Vt=terminal velocity of the droplet            d=diameter of the droplet            g=gravitational acceleration,            Δρ=difference in density between the surrounding fluid and the oil droplet, and            μ=fluid viscosity.                        
Stokes' law is valid when the fluid that the droplets are rising through is characterized by laminar flow. More specifically, the Reynolds Number, which is a ratio between the inertial and viscous forces within a fluid and may be used to determine whether fluid flow is laminar or turbulent, should have a value less than 1:Re=ρfVtd/μ<1                where:                    Re=Reynolds Number, and            ρf=the fluid density.                        
Since the velocity of the droplet depends on the diameter as well, having a small Reynolds Number requires that the diameter of the oil droplet be relatively small for Stokes' law to be valid.
The terminal velocity of small oil droplets rising in a skim tank tends to be very slow, on the order of centimeters per second. Moreover, since the terminal velocity of a rising oil droplet is proportional to the square of its diameter, this means that smaller oil droplets tend to rise at even slower terminal velocities than larger droplets.
In view of the low terminal velocities of oil droplets in a skim tank, SAGD skim tanks are typically large, often exceeding 45′ in diameter and 50′ in height, in order to reduce the mean fluid velocity in the tank for a given inflow rate and create a quiescent environment to allow small oil droplets to separate. To further reduce fluid velocity in the skim tank, conical diffusers are sometimes employed at the fluid inlets, to cause the oil-containing liquid to expand into a wider cross-sectional area and thereby decelerate to a lower velocity immediately before entering the tank. Although barriers such as mesh gratings can be used in other contexts to reduce fluid velocities, such barriers are not suitable for oil-containing liquid due to fouling concerns.
In addition, to reduce “short-circuiting,” meaning the tendency of liquid to flow directly from the inlets to the exhaust without spending sufficient time in the tank to allow oil to separate, skim tanks often include a conical barrier or diverter near the bottom of the tank, with the top of the cone above the entrance to the exhaust pipe through which water exits, and the edges of the cone extending downward to near the outer perimeter of the tank. The conical diverter increases the minimum distance that liquid must travel between the inlet and exhaust, thereby increasing the minimum residence time of the liquid in the tank.
However, existing skim tanks are relatively inefficient, and their exhausted water often contains an undesirably large amount of residual oil. This not only represents lost oil production revenue, but can also lead to significant and costly downstream problems such as those mentioned above, including the fouling of ion exchange resins in water softeners and damage to steam generator tubes for example, which in turn can necessitate significant and costly production shutdowns for maintenance.